Heretofore, there has been employed an electrolytic surface-roughening method as one of surface-roughening methods for a support of a planographic printing plate. However, when trying to obtain the surface roughness necessary for a support of a planographic printing plate only through electrolytic surface-roughening, the roughened surface has not been uniform sufficiently. In the case of electrolysis of the support in an electrolytic solution mainly containing hydrochloric acid, in particular, too large pits exceeding 10 .mu.m in terms of an opening size have tended to be generated, flat portions have remained unroughened without generation of relatively large pit having an opening size of 3-10 .mu.m, and only an unevenly roughened surface has been obtained.
In the case of electrolysis of the support in an electrolytic solution mainly containing nitric acid, on the other hand, too large pits exceeding 10 .mu.m in terms of an opening size has hardly been generated, the distribution of the opening size has focused on a range of 1-3 .mu.m, and generation of pits with an opening size of 1 .mu.m or less has been only a little. Therefore, the resulting support tends to soil a blanket of a printing machine, though the roughened surface has been uniform.
To solve the problems mentioned above, there is employed a method wherein relatively large pits are formed through mechanical surface-roughening, while small pits with an opening size of about 1 .mu.m are formed through electrolytic surface-roughening. However, pits or swells formed through the mechanical surface-roughening corresponds to pits having an opening size of about 10 .mu.m, and it has been impossible to form a pit having an opening size ranging from about 3 .mu.m to 6 .mu.m.
Further, Japanese Patent Examined Publication No. 98429/1995 discloses that generation of too large pits having an opening size of 10 .mu.m or more can be eliminated by providing at least two standstills during electrolytic processing, in the case of the electrolytic surface-roughening. However, in the method disclosed by Japanese Patent Examined Publication No. 98429/1995, it is still impossible to obtain sufficient uniformity, and properties to minimize both dot gain at high fineness and ball-point pen damage have not been satisfactory.
The present inventors have found, after perceiving split processing for the electrolytic surface-roughening and conducting various studies, that what is closely related to uniformity of grain is not the number of the standstills but an average quantity of electricity to be applied during one of electrolytic processing steps, and that no effect of uniformalization is obtained when a period of time for the standstill of electrolytic processing is 0.5 sec or less, and the effect of uniformalization can be obtained even when an electric current for the electrolysis is completely cut for the period of standstill. They have further found that the uniformalization can provide a remarkable effect for an improvement in properties to minimize both dot gain at high fineness and a ball-point pen damage. Thus, they have achieved the present invention.